1. Field of Invention
This invention relates in general to satellite navigation systems and in particular to mitigating the effects of narrowband interference in Global Positioning System (“GPS”) receivers.
2. Related Art
One major advantage of a spread spectrum system such as a satellite position system receiver (e.g., GPS receiver), is its ability to reject interference such as wide band interference. This ability is due to the correlation of a wide band interference signal with the reference code that reduces the power density of the interference and its effect on the resulting signal. However, this approach is also known to make the GPS receiver susceptible to narrow band interference. An example of this type of interference is commonly called continuous wave carrier (CW) interference. In fact, the narrower the bandwidth of the interference signal the less is the ability of the GPS or spread spectrum receiver to reject it. The failure to reject this narrowband interference results in degraded performance such as degraded receiver sensitivity.
Past methods of suppressing narrowband interference have included excision of interference in the frequency domain using OFFT (Overlapped Fast Fourier Transform) techniques or using ATF (adaptive transversal filter) techniques in the time domain. Use of frequency domain techniques results in faster response time, which approximately equals the FFT duration, but also degrades the GPS signal. This degradation even occurs when no interference is present in the received signal, thus lowering the receiver's sensitivity.
In contrast to frequency domain techniques, time domain techniques suffer from poorer interference suppression capability for higher power interference. This results because of severe distortion to the spread spectrum or GPS signal but it incurs no loss when no interference is present. The number, frequency and power of the narrowband interference may also dictate which technique to use for the optimal interference suppression performance under the operating environment. For example, the presence of a large number of interferences just outside the GPS signal band may require using just a filter in the time domain to reject the out of band interference so as to avoid the inherent degradation to the signal introduced by the finite duration of the FFT. On the other hand, the presence of a few powerful interferences inside the GPS signal band would require the use of the OFFT for its better interference suppression capability. The problem is further complicated by the time varying nature of the interference source, requiring GPS receivers to be able to quickly adapt its narrowband interference technique in response to the changing environment. In addition to using OFFT for interference suppression, past methods have also used OFFT to detect the presence of narrowband interference.
To improve GPS receiver sensitivity, relatively low power narrowband interference has to be detected and mitigated. Detecting low level narrowband interference using OFFT requires that the OFFT process runs longer and has more frequency bins. However, for fast interference mitigation response time in a dynamic environment, it is desirable to shorten the FFT duration. Therefore, the conflicting requirements when using OFFT for both detection and mitigation often result in less than optimal performance for both.
Therefore, there is a need for a system and method capable of mitigating the effects of narrowband interfering signals in a GPS Receiver.